[PATCH v2 3/4] iomap: don't lose a fiemap iteration error when emitting the last extent

[Andrea Parri <parri.andrea@gmail.com>]

iomap_fiemap() emits extents one behind: iomap_fiemap_iter() flushes the
previous extent and remembers the current one, and the remembered extent
is written with FIEMAP_EXTENT_LAST after the iteration loop.

That final flush overwrites ret, so when ->iomap_begin() fails partway
through the iteration the error is replaced by the result of
iomap_to_fiemap() (zero on success) and iomap_fiemap() returns success
with a truncated extent list whose last entry is wrongly marked as the
last extent in the file.  The pre-iomap_iter code returned the error
from inside the loop, before flushing the pending extent.

Check for the iteration error before flushing the pending extent, so
that real errors are propagated and only a successful iteration emits
the final FIEMAP_EXTENT_LAST extent.  -ENOENT (no mapping) is still not
an error, and the pending extent is still emitted in that case.

Fixes: 7892386d3571 ("iomap: switch iomap_fiemap to use iomap_iter")
Cc: <stable@vger.kernel.org> # v5.15
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 fs/iomap/fiemap.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/iomap/fiemap.c b/fs/iomap/fiemap.c
index d11dadff82865..54b824b7edb5c 100644
--- a/fs/iomap/fiemap.c
+++ b/fs/iomap/fiemap.c
@@ -76,15 +76,15 @@ int iomap_fiemap(struct inode *inode, struct fiemap_extent_info *fi,
 	while ((ret = iomap_iter(&iter, ops)) > 0)
 		iter.status = iomap_fiemap_iter(&iter, fi, &prev);
 
+	/* inode with no (attribute) mapping will give ENOENT */
+	if (ret < 0 && ret != -ENOENT)
+		return ret;
+
 	if (prev.type != IOMAP_HOLE) {
 		ret = iomap_to_fiemap(fi, &prev, FIEMAP_EXTENT_LAST);
 		if (ret < 0)
 			return ret;
 	}
-
-	/* inode with no (attribute) mapping will give ENOENT */
-	if (ret < 0 && ret != -ENOENT)
-		return ret;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(iomap_fiemap);
-- 
2.53.0